1. Field of the Invention
The present invention relates to a charging device including a mechanism for moving a cleaning member configured to clean a charging electrode in a corona charging device in a longitudinal direction by a screw.
2. Description of the Related Art
An electrophotographic image forming apparatus using a corona charging device configured to apply a voltage to a charging wire to charge a photosensitive member by corona discharge has been known. When the charging wire is contaminated with dust or toner, a surface potential of the photosensitive member becomes non-uniform, causing image failure called image unevenness, in addition to a decrease in charging efficiency of the photosensitive member.
A corona charging device including a mechanism for moving a cleaning pad for cleaning a charging wire in a longitudinal direction with a screw, to clean the charging wire has been discussed. In a configuration in which the cleaning pad cleans the charging wire, charging failure occurs when the cleaning pad stops at the center of the charging wire (a photosensitive member charging area).
Therefore, in a configuration in which a carriage, which engages with a screw without jumping, moves a cleaning pad in a longitudinal direction, a position detection sensor for detecting that the carriage is positioned at an end of the screw can be provided at the end of the screw, to control the carriage to stop at the end of the screw. When the position detection sensor is provided, however, the cost unfavorably rises.
On the other hand, Japanese Patent Application Laid-Open No. 09-119499 discusses a configuration in which a spring urges, toward the screw, a member, which engages with a groove of a screw, in a carriage configured to move a cleaning pad, to allow the member to jump when a predetermined load or more is applied to move the carriage. In a configuration in which the carriage can jump over the entire length of the screw in a longitudinal direction, when the carriage abuts on a wall surface at an end of the screw so that the predetermined load or more is applied thereto, the carriage can jump at the end of the screw to stop without using a sensor. More specifically, if the screw is turned while the carriage contacts the wall surface at the end of the screw, a protrusion, which engages with concave and convex portions of the screw, provided in the carriage jumps a convex portion of the screw, to stop the carriage at the end of the screw.
On the other hand, a charging performance of the corona charging device is required to be improved to improve the productivity of a print product to be output by an image forming apparatus. To improve the charging performance, a voltage to be applied to the charging wire may be increased, or a plurality of charging wires for charging a photosensitive member may be used. When the voltage to be applied to the charging wire is increased to improve the charging performance, the degree of dirt of the charging wire is aggravated in addition to an increase in a required creeping distance (a distance in a longitudinal direction of the corona charging device). In a configuration in which the plurality of charging wires is used, a plurality of cleaning pads is similarly required. When the length and the number of charging wires and the dirt that adheres to the charging wires are increased to improve the productivity of the print product, a torque required to move the cleaning pads for cleaning the charging wires in a longitudinal direction is increased.
In a configuration in which a carriage jumps at an end of a screw while improving a charging performance of a corona charging device to enhance productivity, the following problems occur. Specifically, when a force generated when a cleaning pad serving as a cleaning member is moved in a longitudinal direction is increased due to an increase in the dirt that adheres to a charging wire serving as a charging electrode, a force required for the carriage to move in the longitudinal direction is also increased. When a force to move the carriage in the longitudinal direction becomes predetermined value or more due to the dirt, the carriage jumps in an unintended place (a place other than the end of the screw).
Particularly when a protrusion of the carriage jumps from a helical groove of the screw, and the carriage stops at the center of the corona charging device (a site where an area, where an electrostatic image is to be formed, of a photosensitive member is charged), image failure called a white streak or a black streak occurs.